1. Field of the Invention
This invention relates in general to a method and apparatus for implementing a redundant or back-up transmission capability for the automatic frequency control pilot signal in a satellite communications system.
2. Description of the Prior Art
In communications systems involving the transmission and reception of many messages using frequency division multiplexing, the carrier frequencies are often subject to frequency translations which introduce frequency errors in the received signals, thereby complicating the detection and demodulation of the desired channels. In the prior art, this problem is typically solved by adding a single unmodulated pilot carrier to the FDM spectrum at the transmitter. This pilot signal is then compared to a local replica of the pilot frequency at the receiver, and an error signal is generated. The latter is fed back to a local AFC translator in the receiver which acts to subtractively or otherwise compensate for said frequency error.
When the communications system includes a satellite relay, the carriers which are being multiplexed may originate from many different earth or ground stations. Frequency errors in the satelite relay frequency translation caused by uncertainties in the up converted oscillator, doppler shift due to satellite motion, variations in the satelite transponder oscillator, etc., may be compensated for at each receiving earth station by comparing the frequency of a pilot signal transmitted by one of the stations with a local replica of the pilot signal as described above. However, if the station transmitting the pilot signal should suffer a failure which results in the loss of the pilot signal transmission, the entire system will suffer frequency drift and become inoperative. In such a system it is therefore not desirable to have the reliability of the entire system depend on the operability of just one station in the system.
This problem is overcome in the prior art by assigning a pilot signal back-up function to a second earth station. This station monitors the presence of the AFC pilot signal in the received spectrum, and if the pilot signal is lost for a predetermined time the second station itself then transmits the pilot signal to thus maintain AFC continuity throughout the system. A further problem can arise, however, in that when the pilot signal is lost the second station may be uncertain as to whether the failure is in the transmitting station or in the second station itself.